GB2162515A - Liquid crystal esters - Google Patents

Abstract

Esters having a general formula R1-A1-Ph1-E-A2-Ph1-R2 where each of A1 and A2 independently represents phenyl or cyclohexyl, E represents carbonyloxy or oxycarbonyl, R1 and R2 independently represent alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl, alkoxy carbonyloxy, cyano, F, Cl, CF3 or H, and Ph, represents 1,4- phenyl-ene optionally carrying at least one lateral F or Cl provided, that (i) not more than one of R1 and R2 represents cyano, F, Cl, CF3 or H and (ii) at least one phenyl group carries at least one lateral F or Cl, are useful in liquid crystal materials and are particularly suitable for use in a two-frequency switching devices.

Description

SPECIFICATION Esters and their use in liquid crystal materials and devices The present invention relates to esters and their use in liquid crystal materials and devices.

The use of liquid crystal materials to exhibit electro-optical effects in display devices such as digital calculators, watches, meters and simple word displays is now well known. However, known liquid crystal materials are not ideal in all respects and a considerable amount of work is currently being carried out in the art to improve their properties.

Liquid crystal materials normally consist of specially selected mixture compositions and improved materials are obtained by forming new mixtures having an improved combination of properties, eg by the provision of new compounds for incorporation in the mixtures. Preferably, such compounds are nematogenic or chiral nematogenic in nature.

According to the present invention a novel ester has a formula: R,-A,-Ph,-E-A2-Ph,-R2 wherein: each of A, and A2 independently represents Ph, or Cy; E represents -CO.O- or -O.OC-; R, represents alkyl, or, where A, represents Ph1, alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl, alkoxycarbonyloxy, cyano, fluore, chloro, CF3 or hydrogen; R2 represents alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl, alkoxycarbonyloxy, cyano, fluoro, chloro, CF3 or hydrogen; Cy represents a trans-l ,4-disubstituted cyclohexane ring; and Ph, represents a 1,4-disubstituted benzene ring optionally carrying at least one lateral halogen substitutent selected from F and Cl; with the provisos that: (i) not more than one of R, and R2 represents cyano, fluoro, chloro, CF3 or hydrogen, (ii) at least one Ph, group carries at least one lateral halogen substituent selected from F and Cl.

In the accompanying drawings: Figure 1 is a list of chemical formulae representing preferred sub-classes of compound falling within the class of Formula Figures 2 to 5 are lists of compounds which may be admixed with the compounds of Formula 1.

Figures 6 to 10 are flow diagrams illustrating examples of preparative routes by which intermediates used in the preparation of the compounds of Formula I may be produced.

Examples of certain preferred sub-classes falling within the overall scope of the class represented by Formula I are shown in Fig. 1.

In Fig. 1 R, and R2 are as defined above for Formula I and W, X, Y and Z are each selected from H, F and Cl, at least one per molecule being F or Cl. In Fig. 1 R, is preferably alkyl or alkoxy and R3 is preferably cyano.

In the compounds of Formula I where an alkyl group is contained within the group R1 and/or the group R3 it preferably has from 1 to 1 2 carbon atoms, and it is preferably an n-alkyl group (where it contains more than 2 carbon atoms). An alkyl group contained within the group R, and/or the group R2 may alternatively be a chiral alkyl group having from 4 to 1 2 carbon atoms, eg (+ )-2-methylbutyl.

Preferably, R, is alkyl or alkoxy and has between 1 and 8 carbon atoms and R2 is cyano.

Generally speaking, the compounds of Formula I are nematogenic or chiral nematogenic (where R, or R2 is chiral) showing a high clearing point (liquid crystal to isotropic liquid transition temperature), a good chemical and photochemical stability. They may show a positive dielectric an isotropy (at low frequencies) when one of the groups R, or R2 is halogeno or cyano.

They may also show other advantageous properties such as (i) a reduction in the tendency to form smectic phases by themselves and in admixture with nematogenic compounds of smaller dielectric anisotropy compared with the tendency to form such phases as shown by unfluori nated compounds of comparable structure and (ii) a change in dielectric anisotropy sign at moderate frequencies.

The compounds of Formula I are therefore useful compounds of liquid crystal compositions for electro-optical displays.

As the compounds of Formula I normally show a positive dielectric anisotropy when either R, or R2 is cyano or halogeno they may be added to other liquid crystal materials of positive dielectric anisotropy in order to produce a mixture having amongst other things a suitable liquid crystal temperature range, dielectric anisotropy and viscosity. As is well known to those skilled in the art the dielectric an isotropy of the liquid crystal material is necessary to give electro optical operation and its sign (for a give frequency) is chosen according to the kind of electrooptical device in which the material is to be used and the viscosity is necessarily minimised to reduce device switching speeds.

Compounds of reasonably low melting point are preferred as high dielectric an isotropy components added to compounds of Formula I. For example, the low melting compounds of the known classes listed in Fig. 2 are suitable as positive materials for admixture with the compounds of Formula I. In Fig. 2 each R is independently n-alkyl or n-alkoxy and each R1 is independently n-alkyl.

As noted above, the compounds of Formula I may have added to them small dielectric anisotropy compounds, eg to reduce mixture melting point, viscosity or to improve multiplexibility. The classes listed in Fig. 3 are examples of such small dielectric anisotropy compounds.

In Fig. 3 each R is independently n-alkyl or n-alkoxy; each RA is independently n-alkyl; each R' is independently n-alkyl, n-alkoxy or hydrogen; X= H or F; and Q = halogen, eg Cl or F.

In general, one or more compounds of Formula I is likely to be added to one or more compounds of Formula Illa to Illi optionally together with one or more compounds of Formula Ila to lli to form suitable liquid crystal mixtures.

Additional high clearing point compounds may be included in such mixtures eg one or more compounds selected from the classes shown in Fig. 4.

In Fig. 4 R, RA, and X are as specified above (each one being independently selected).

Other specific known additives, eg low melting chiral additives, such as

where Rc = ( + )-2-methylbutyl and RD = ( + )-2-methylbutoxy, may be incorporated in the mixture where required.

One or more of compounds according to Formula I may be used in any of the following applications (where material having a positive dielectric anisotropy is referred to as 'positive' material.

(i) as one or more components of a positive nematic material for use in twisted nematic effect devices including multiplexed devices; an example of such a device is given below; (ii) as one or more components of a positive nematic material, preferably also with a pleochroic dye, for use in Fréedericksz effect devices (positive nematic type) in which the molecular arrangement may be changed from the homogeneous texture (OFF state) to the homeotropic texture (ON state) by an electric field;; (iii) as one or more components of a positive material which is a long helical molecular pitch (eg 3 m) cholesteric (chiral nematic), preferably together also with a pleochroic dye, in cholesteric-to-nematic phase change effect devices (negative contrast type) in which the molecular arrangement may be changed from a scattering focal conic texture (OFF state) to a clear homeotropic texture (ON state) by an electric field; (iv) as one or more components of a positive nematic material in two frequency switching effect devices (which may be twisted nematic effect devices) in which the dielectric anisotropy of the material may be changed from (at low frequency) positive (OFF state) to negative (ON state) by the application of a high frequency electric field.

(v) As one or more components of a device as described in UK Patent Application No 8317355 Publication No GB 2123163A.

The construction and operation of the above devices and the general kinds of material which are suitable for use in them are themselves known.

Compositions incorporating the compounds of Formula I for use in the above applications may for example contain the following components.

Component A: one or more compounds of Formula I optionally plus one or more compounds of Formula Ila to Ili; Component B: one or more compounds of Formula Illa to lllq; Component C: one or more compounds of Formula IVa to IVo; Component D: one or more low melting chiral additives.

For the twisted nematic effect and Freedericksz (positive nematic) effect the following percentages of the various components may be used in the material (the overall sum of the percentages adding to 100%).

Component A: 5 to 95% by weight (typically 5 to 75% by weight) Component B: 0 to 90% by weight (typically 5 to 25% by weight) Component C: O to 30% by weight (typically O to 20% by weight) Component D: O to 5% by weight (typically O to 1 % by weight) For the Fréedericksz (positive nematic) and phase change (negative contrast type) effects a pleochroic dye forming from + 0.5 to 15% of the overall mixture is preferably added to the liquid crystal material. Suitable dyes are described in published UK Patent Application Nos 2081736A, 20821 9A and 2093475A. Typically, each dye compound incorporated forms 1 to 3% by weight of the overall mixture.

Liquid crystal mixtures including compounds of Formula I may be formed in a known way, eg simply by heating the constituent compounds together in the correct weight proportion to form an overall isotropic liquid (eg at about 100"C).

To provide a more general example of a mixture embodying the invention at least one compound according to Formula I above may be mixed together with one or more compounds in any one or more of the known families listed in Fig. 5 for use in one or more of the applications given above (the actual application(s) depending on the mixture's properties).

In Fig. 5 X is a 1,4-phenylene group, a 4,4'-biphenylyl group, a 2,6-naphthyl group or a trans-1,4-disubstituted cyclohexane ring, and Y1 is CN, or R' or halogen or CO.O-X-Y1 where Y1 is CN, or R' or OR'; where R and R' are alkyl groups. The compound may alternatively be a derivative of one of these wherein H is replaced by a halogen, eg F, in one of the benzene rings.

Preferably, the compound(s) of Formula I comprises between 5 and 95% by weight of the mixture.

According to the present invention in a second aspect a liquid crystal device includes two dielectric substrates at least one of which is optically transparent, a layer of liquid crystal material sandwiched between the substrates and electrodes on the inner surfaces of the substrates to enable an electric field to be applied across the layer of liquid crystal material to provide an electro-optic effect therein, characterised in that the liquid crystal material consists of or includes a compound according to Formula I above.

The device according to the second aspect may be a twisted nematic effect device, which may or may not be operated in a multiplexed fashion, a cholesteric-to-nematic phase change effect device, a Fréedericksz effect device or a two-frequency switching effect device, all constructed in a known manner or any of the other devices mentioned above. The various ways in which compounds according to Formula I may be used in these devices are outlined above and will be further apparent to those skilled in the art.

The compounds of Formula I are particularly suitable for use in materials (and electro-optical devices incorporating them) whose dielectric an isotropy is switched by a known two-frequency switching effect.

Esters of Formula I may be prepared by preparative routes in which the individual procedures are known per se, the overall route being new.

The generalised route used is preferably Route 1 or Route 2 as follows: Route 1 R,-A,-Ph-CO.OH t Step 1a R1-A1-Ph-CO.CI + HO-A2-Ph-R2 t Step 1b R1-A1-Ph-CO.0-A2-Ph-R2 Route 2 R2-Ph-A2-CO-.OH R Step 2a R1-A1-Ph-OH + R2-Ph-A2-CO.CI > , Step 2b R1-A1-Ph-O.OC-A2-Ph-R2 The starting materials for Route 1 and 2, viz compounds of formulae R1-A,-Ph-CO.OH, R,-A,-Ph-OH, R2-Ph-A2-OH and R2-Ph-A2-CO.OH, are known or may be prepared from known starting materials by well known methods.

The chlorination procedure used in Step 1 a is essentially the same as that used in Step 2a.

Likewise, the esterification procedures used in Step 1 b is essentially the same as that used in Step 2b.

Examples of the preparation and properties of compounds of formula I and the preparation of intermediates used in their preparation, will now be described.

In these examples of the following symbols are used: C-N = crystalline solid to nematic liquid crystal transition temperature ("C); N-l = nematic liquid crystal to isotropic liquid transition temperature ("C).

The following example, Example 1, illustrates Routes 1 and 2 by illustrating Steps 1 a and 1 b of Route 1 given above.

Example 1: Preparation of 2-chlorn-4-cyanobiphenyl-4'-yl 4 '-n-butyl-2-chlorobiphen yl-6carboxylate Step la 4'-n-Butyl-2-chlorobiphenyl-4-carboxylic acid (0.67 9, 2.33 mmol) is dissolved in an excess of thionyl chloride (6 ml). The solution is refluxed for one hour, the mixture cooled, toluene (6ml, dry) added, and the excess of thionyl chloride and toluene removed under reduced pressure.

Further toluene is added and the solution evaporated to ensure complete removal of thionyl chloride.

Step ib The residue from Step 1a is dissolved in dichloromethane (6 ml), the solution cooled in an icebath, and a solution of 2-chloro-4-cyano-4'-hydroxybiphenyl (0.53 g, 2.33 mmol) in dichloromethane (6 ml, dry) and triethylamine (4 ml) is added dropwise. The resulting mixture is refluxed with stirring for three hours. 2M-HCI (10 ml) is slowly added to the cooled reaction mixture, the organic phase separated, washed with water, and dried (MgSO4).The solvent is removed under reduced pressure and the residue taken up in the minimum volume of chloroform, and purified by chromatography on a column of silica gel using chloroform as eluent. Recrystallisation from ethyl acetate gives the required ester, 0.86 g (74%), C-N1 63.5', N-l 221".

Compounds which may be prepared analogously are listed in Tables 1 to 8 as follows.

The following examples illustrate routes for preparing various halogenated intermediates for use in Routes 1 and 2 specified above.

Route A This is as shown in Fig. 6 of the accompanying drawings. An example of carrying out this Route wherein R, is n-butyl is as follows: Step Al This esterification is carried out by the standard method described in Vogel, Textbook of Practical Organic Chemistry, 4th ed, 841 (1978).

Step A2 4-n-Butylaniline (36.91 g, 0.250 ml, commercially available) is dissolved in concentrated hydrochloric acid (88 ml) and water (44 ml), the stirred solution cooled rapidly to 0 C, and the resulting fine suspension of the amine hydrochloride is diazotised by the dropwise addition of sodium nitrite (18.50g, 0.268 mol) in water (36 ml). To this solution is added ethyl 3chlorobenzoate (46.2 g, 0.25 mol), the mixture being maintained at 0 C with vigorous stirring while aqueous NaOH is added dropwise until the emulsion is just alkaline.The stirring is continued for 1 6 hours at room temperature before the organic phase is separated, washed with water and dried (MgSO4). The excess of ethyl 3-chlorobenzoate is removed by distillation under reduced pressure and the final residue distilled at 140 -170 /0.1 mmHg to give a red oil.

This oil, an isomeric mixture of ethyl biphenylcarboxylate esters is hydrolysed by boiling with sodium hydroxide (45 g) in methanol (167 ml) and water (33 ml) for 4 hours. The hydrolysate is diluted with water (500 ml) and acidified with concentrated hydrochloric acid. The precipitated orange solid is filtered off, washed with water and recrystallised from acetic acid: water (4:1) to give colourless needles of 4'-n-butyl-2-chlorobiphenyl-4-carboxylic acid (2.46 g, 3.4%), C-N 119 , N-l 174"C.

Route B 4-Methoxyaniline is commercially available.

This Route is as shown in Fig. 7 wherein Me = methyl and Et = ethyl. An example of carrying out this Route is as follows, wherein R1 = n-butyl.

Ethyl 3-chlorobenzoate is prepared in Route A, Step Al.

Step B1 This step is carried out in a similar way to Step A2.

Step B2 This step is carried out by a standard method (of demethylation) by the use of boron trifluoride in a suitable solvent as described by McOmie, Watts, and West Tetrahedron, 24, 2289 (1968).

Step B3 A solution of 1-iodobutane (1.03 g, 5.6 mmol) in dry butanone (5 ml) is added dropwise in anhydrous conditions to a stirred mixture of 2-chloro-4'-hydroxybiphenyl-4-carboxylic acid (0.63 g, 2.5 mmol) and anhydrous potassium carbonate (1.93 g, 14.0 mmol) in dry butanone (25 ml). The resulting mixture is refluxed in anhydrous conditions for 48 hours. The excess of potassium carbonate is filtered off, the solvent removed, and the residual liquid dissolved in ethanol. This solution is boiled for 2 hours with a solution of potassium hydroxide (0.6 9) in water (3 ml), then diluted with water (50 ml) and acidified with dilute hydrochloric acid.The colourless precipitate is filtered off, washed with water and then with petroleum ether (bp 40-60"), dried in vacuo, and recrystallised from acetic acid: water (6:1 V/V) to give colourless crystals of 4'-n-butoxy-2-chlorobiphenyl-4-carboxylic acid, 0.41 g (53%), C-N 164", N-l 196'.

Route C This is as shown in Fig. 8 wherein Me = methyl. An example of carrying out this Route is as follows, wherein R, = n-butyl.

Step C1 Copper(l) chloride (11.889, 0.12 mol) and copper(ll) chloride (16.15 g, 0.12 mol) are added to 4-methoxyaniline (98.4 g, 0.8 mol) dissolved in hydrochloric acid (1000 ml, 7 M). The stirred reaction mixture is placed under an atmosphere of nitrogen and is heated until gently reflexing. At this stage, chlorine gas is slowly bubbled into the refluxing solution for 1 8 hours.

The reaction mixture is allowed to cool, and the free amine liberated by the addition ofpotassium hydroxide (672 g) in water (1400 ml). The mixture is steam-distilled, 2.5 1 of distillate being collected. The steam distillate is extracted by ether (1 X 500 ml, 2 X 250 ml), the combined extracts dried (MgSO4), the solvent removed and the residue distilled, the fraction distilling at 60-65"/0.2 mmHg being 2-chloro-4-methoxyaniline, 23.1 g (18%; purity 95%).

Step C2 This step is carried out in the same way as Step A2 above.

Step C3 This step is carried out in the same way as Step B2 above.

Step C4 This step is carried out in the same way as Step B3 above.

Route D This Route is shown in Fig. 9 wherein Me = methyl. An example of this Route is as follows.

The starting material 2-chloro-4'-methoxylbiphenyl-4-carboxylic acid is prepared in Step A2.

Step D1 Conversion of the acid into the amide is carried out by the standard method described in Vogel, Textbook or Practical Organic Chemistry, 4th ed, 111 6 (1978).

Step D2 This dehydration of the acid amide to the nitrile is carried out by the method using thionyl chloride in dimethylformamide as described in published UK Patent Application No. GB 2 063 288A.

Step D3 This Step is carried out in the same way as Step C3 above.

Route E This Route is shown in Fig. 10 wherein Me = methyl. An example of this Route is as follows.

The starting material 2'-chloro-4'-methoxybiphenyl-4-carboxylic acid is prepared as in Step C2.

Step El This step is carried out in the same way as Step D1.

Step E2 This step is carried out in the same way as Step D2.

Step E3 This step is carried out in the same way as Step D3.

Examples of compounds which may be prepared in the same manner as Example 1 are listed in Tables 1 to together with their transition temperatures.

TABLE 1: COMPOUNDS OF FORMULA:

R X Y C-N N-l C4H9 H Cl 132 314 C4H9 CI H 112 305 C7H15 Cl H 106 269.5 TABLE 2: COMPOUNDS OF FORMULA:

R X Y C-N N-I C4H9 H Cl 124 309 C4H9 Cl H 118 300.5 C7H15 H Cl 118 286.5 C7H15 Cl H 109 274.5 TABLE 3: COMPOUNDS OF FORMULA:

R X Y C-N N-l C4H9 H CI 166 343 C4H Cl H 127 329 TABLE 4: COMPOUNDS OF FORMULA:

R X Y C-N N-l C4H9 H Cl 134.5 308 TABLE 5: COMPOUNDS OF FORMULA:

R X Y C-N N-l C4H9 H Cl 153.5 329 C4H9 Cl H 137 332.5 TABLE 6:COMPOUNDS OF FORMULA:

R C-N N-l C4H9 163.5 221 TABLE 7: COMPOUNDS OF FORMULA:

R W X Y Z C-N N-l C4H9 H Cl H Cl 155 252 C4H9 H Cl CI H 156 232.5 C4H9 Cl H H Cl 149 254 C4H9 Cl H CI H 148 242 TABLE 8: COMPOUNDS OF FORMULA:

W X Y Z C-N N-l H F H H 132 374 H F CI H 120.5 H F H Cl 132 Examples of materials embodying the present invention suitable for use in a two-frequency switching device are Mixtures 7 and 9 specified in published UK Patent Specification No.

2085910A with the component of formula:

replaced by a compound of Formula I specified herein, eg the compound of formula:

Another example of a liquid crystal material embodying the present invention is the following mixtures:

Where the following combinations of R and R1 may be used: R = n-C3H7, R1 = n-C5Hl,; R = R1 = n-C5H"; R = n-C5H11, R' = n-C7H,s. This mixture was found to be suitable for use in a two-frequency switching device. At 21 the mixture had the following properties: Fo = 1.7 KHz; #1 = 4.3; ##LF(100H2) = +0.9; ##HF(20 KH,)= -1.3.

Fo is the frequency of electrical signals applied to the liquid crystal mixture at which the dielectric anisotropy of the mixture changes from + ve to - ve. 1 is the dielectric constant of the mixture measured perpendicular to the average molecular axis of the aligned phase.

#ii is the dielectric constant of the mixture measured parallel to th average molecular axis of the aligned phase.

ALF = e11-e1 at frequencies very much below Fo.

33HF = = ll-1 at frequencies very much above Fo.

Claims (11)

1. An ester having formula R1-A1-Ph1-E-A2-Ph1-R2 wherein each of A, and A2 independently represents Ph or Cy; E represents -CO.O- or O.OC-; R, represents alkyl, or where A1 represents Ph1 alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl, alkoxycarbonyloxy, cyano, fluoro, chloro CF3 or hydrogen; R2 represents alkyl, alkoxy, alkoxy substituted alkyl, alkanoyl, alkanoyloxy, alkoxycarbonyl, alkoxycarbonyloxy, cyano, fluoro, chloro, CF3 or hydrogen; Cy represents a trans 1,4-disubstituted cyclohexane ring; and Ph1 represents a 1,4-disubstituted benzene ring optionally carrying at least one lateral halogen substituent selected from F and Cl; with the provisos that: (i) not more than one of R1 and R2 represents cyano, fluoro, chloro, CF3 or hydrogen, (ii) at least one Ph group carries at least one lateral halogen substituent selected from F and Cl.

2. An ester as claimed in claim 1 and having the formula:

where R1, A1 and Ph1 are as defined in claim 1.

3. An ester as claimed in claim 2 and wherein the group R1 is an n-alkyl or an n-alkoxy group having from 1 to 1 2 carbon atoms.

4. An ester as claimed in claim 2 or claim 3 and having the formula:

where R is alkyl or alkoxy and X and Y are either H or Cl, at least one being Cl.

5. An ester as claimed in claim 1 and having the formula:

wherein R is alkyl or alkoxy and X and Y are either H or Cl, at least one being Cl.

6. An ester as claimed in claim 1 and having the formula:

where R is alkyl or alkoxy and X and Y are either H or Cl, at least one being Cl.

7. An ester as claimed in claim 1 and having the formula:

where R is alkyl or alkoxy.

8. An ester as claimed in claim 1 and having the formula:

where R is n-alkyl or n-alkoxy and W, X, Y and Z are H, F or CC at least one being F.

9. An ester as claimed in claim 1 and in which R, and/or R2 contains a chiral alkyl group.

1 0. A liquid crystal material comprising a mixture of compounds at least one of which is an ester as claimed in claim 1.

11. A liquid crystal electro-optical device incorporating a liquid crystal material as claimed in claim 10.